White House officials have homed in on President Donald Trump’s Washington transition headquarters as a likely location where chief of staff John Kelly's personal cellphone could have been compromised in late 2016, twoU.S. officials familiar with the matter said.

Kelly, who first served as Trump’s secretary of homeland security, reported the phone had stopped working properly in December after he entered the transition office space, which was made available by the General Services Administration, the officials said. Kelly said it functioned well before that time.

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Although many of Trump’s high-profile meetings with lawmakers and potential Cabinet members before his inauguration occurred in New York at Trump Tower, much of his transition staff worked out of the office space about three blocks from the White House.

Officials have tried to determine whether Kelly signed onto an insecure wireless network there or whether a hacker, foreign government or some other outside force could have accessed the phone there.

"That's where it seems to have started," one White House official with knowledge of the review said. The official said the transition building was only one possible site for the breach. Officials have not ruled out other possibilities, such as foreign trips before Kelly joined the administration, though they have not seen evidence for that.

The White House declined to comment.

Pinpointing how Kelly’s phone was compromised could help determine who might have been behind the breach or whether other staffers could have been affected. There has been no indication at this point that any other staffers’ devices were compromised, and it is not clear what data, if any, was accessed on Kelly’s personal phone.

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Nonetheless, the White House has weighed new rules for personal devices, including banning them from the president’s residence and the West Wing, two officials said, though no rules have been finalized. The White House recently added more secure lockers to the building for storing such devices, though the White House has said it was not related to the incident with Kelly’s phone.

POLITICO reported Thursday that White House technical support staff determined late this summer that Kelly's personal cellphone had been compromised and that he turned it in to the White House. He no longer has possession of the device.

It remains unclear why Kelly hung onto his personal phone from December, when he believed it stopped working well, until August. Staffers in the White House questioned Department of Homeland Security officials after Kelly turned in the phone about why it took so long for the problem to be caught, two U.S. government officials said, though the conversation was not described as adversarial.

Many aides have continued to use personal phones despite the warnings, and they have chafed at previous attempts to quell their use. The National Security Agency warned White House staffers during the transition not to use personal devices or email. One fear is that the cameras or microphones on personal cellphones could be turned on without the owner’s knowledge, turning them into clandestine listening devices.

Numerous White House aides also have used personal email for government business. Jared Kushner and Ivanka Trump, both of whom are top advisers in addition to being members of the president’s close family, have used private email addresses that they set up during the transition, including a shared account they used for scheduling.